Effect of Molar Mass on Critical Specific Work of Flow for Shear-Induced Crystal Nucleation in Poly (l-Lactic Acid)

Abstract

The concept of specific work of flow has been applied for the analysis of critical shearing conditions for the formation of crystal nuclei in poly (l-lactic acid) (PLLA). Systematic variation in both time and rate of shearing the melt in a parallel-plate rheometer revealed that these parameters are interconvertible regarding the shear-induced formation of crystal nuclei; that is, low shear rate can be compensated for by increasing the shear time and vice versa. This result supports the view that critical shearing conditions can be expressed by a single quantity, providing additional options for tailoring polymer processing routes when enhanced nuclei formation is desired/unwanted. Analysis of PLLA of different mass-average molar masses of 70, 90, 120, and 576 kDa confirmed improved shear-induced crystal nucleation for materials of higher molar mass, with critical specific works of flow, above which shear-induced nuclei formation occurs, of 550, 60, 25, and 5 kPa, respectively.

Thermal Stability of Shear-Induced Crystal Nuclei of Poly(l-lactic acid)

“…In the center of the disc, large spherulites as in the non-sheared sample grew, while at a distance larger than that indicated with the white dashed circle or line in the top and center images, respectively, the structure appeared much finer. The distance from the center where the structure changes (about one-fourth of the radius) provides information about the critical shearing rate to generate additional nuclei, which, in this particular case (135 °C, 10 s shearing time), is about 0.25 s –1 , being in accord with earlier research . With increasing distance above this threshold, the shear rate increases further to 1 s –1 at the edge, and consequently, the structure becomes increasingly finer due to the continuous increase of the nuclei number.…”

“…The distance from the center where the structure changes (about one-fourth of the radius) provides information about the critical shearing rate to generate additional nuclei, which, in this particular case (135 °C, 10 s shearing time), is about 0.25 s −1 , being in accord with earlier research. 53 With increasing distance above this threshold, the shear rate increases further to 1 s −1 at the edge, and consequently, the structure becomes increasingly finer due to the continuous increase of the nuclei number.…”

“…43−48 In particular, it is obvious that the lifetime of oriented structures does not correlate with the reptation time of the PLLA grade used in the present work. Based on the cross-over frequency of the storage and loss modulus, the relaxation time at 135 °C is less than 1 s, 53 which is expected much shorter between 180 and 190 °C. As such, the long lifetime of crystallization precursors in the melt may arise from (temperature-dependent) molecular interactions, which are suggested to be identified in future research, for example, by infrared spectroscopy as demonstrated in the literature.…”

“…The stability of the remaining structures in the melt seems to be caused/controlled by molecular interactions rather than by slow diffusion of the chain segments toward a random-coiled structure since the longest relaxation time (reptation) in the melt is much less than 1 s. 53 This is several orders of magnitude shorter than the observed lifetime of the ordered structures, being in the range of minutes and hours, depending on temperature.…”

“…However, there still seems to be a lack of knowledge to establish a correlation between the chemical architecture of the macromoleculesfor example, from the points-of-view of the chain stiffness or intermolecular forcesand the critical condition to dissolve/destroy these nuclei. With the present work, we anticipate enhancing the database about the stability/lifetime of shear-induced nuclei in polymers by analysis of poly ( l -lactic acid) (PLLA), with the critical shearing conditions to obtain additional nuclei evaluated recently, , applying the concept of the specific work of flow. − With the experimental approaches applied in this work, a direct comparison of the thermal stability of self-nuclei and shish-like nuclei is attempted.…”